1. Contribution from the Department of Plant Sciences, Weizmann Institute of Science, Rehovot, Israel, Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel, Department of Chemical Services, Weizmann Institute of Science, Rehovot, Israel, Ontario Cancer Institute, Princess Margaret Hospital, Toronto, Canada, and Faculty of Biotechnology, Jagiellonian University, Krakow, Poland